1. Technical Field
The technology of this disclosure pertains generally to band pass filtering in RF transmission, and more particularly to incorporating an FIR filter into Envelope Delta-Sigma Modulation (EDSM) to create band pass filtering with Channelized Active Noise Elimination (CANE).
2. Background Discussion
In designing bitstream transmitters, a problem arises as high sampling rates spread the bandwidth of the signal to be amplified, making it problematic for matching impedance at the power amplifier (PA). In these bitstream approaches the generated quantization noise not only increases receiver noise figure, but also impacts even remote frequency bands. In addition, insertion loss arises in the output network which can significantly degrade power efficiency. Dynamic impedance arises as a result of interaction between the PA and the output network which can lead to excessive signal distortion and power dissipation.
Switching mode power amplifiers can achieve high power efficiency by operating in the saturation region. These PAs are well-suited for the amplification of constant envelope signals but not signals with high Peak-to-Average Power Ratio (PAPR) as the linearity of modulation suffers from the saturation. Bitstream modulated RF transmitters based on Delta-Sigma modulations, such as Low Pass Delta-Sigma Modulation (LPDSM) or Envelope Delta-Sigma Modulation (EDSM) involves the oversampling and noise shaping to convert non-constant envelope to constant envelope and employs an output filter to remove the out-of-band quantization noise and recycle the noise power back to DC power supply. Therefore, in these approaches high power efficiency can be obtained without corrupting the signal integrity.
However, to obtain the desired Adjacent Channel Power Ratio (ACPR), the transmitter requires either high oversampling delta-sigma modulation to shape the quantization noise further from signal band, or utilize a high quality factor output filter. Unfortunately, the former raises the complexity of the modulator design while the latter usually results in a bulky and lossy filter. Even though the quantization noise is near, but not immediately adjacent to the signal, it may still be located in the filter pass band.
To solve this problem, a Finite Impulse Response (FIR) filtering technique has been employed to suppress quantization noise in the bitstream modulated transmitters. The essential idea is to leverage the combining effect of multiple channel power amplifiers to suppress the output noise while the high operating efficiency of power amplifier in each channel is maintained. The FIR structure can either provide band pass filtering or low pass filtering. The band pass filtering with FIR utilizes the higher order pass band of the filter to suppress the quantization noise of digitized Radio Frequency (RF) signal with a pass band at the signal carrier frequency.
It will be noted that LPDSM with FIR bandpass filtering has been demonstrated. While in the literature it has been proposed to filter the bitstreams generated by EDSM with the band pass FIR, where the carrier frequency selection is bound by the delay time selection, with broad bandwidth delay lines being required, due to the fact that RF delay lines are used in that approach. In other proposals a polar transmitter with Delta-Sigma Modulated power supply with low pass FIR is described. However, in that approach only the quantization noise in the envelope path is suppressed, which raises the spectrum regrowth problem in the final modulated RF signal.
Accordingly, a need exists for a Finite Impulse Response (FIR) filtering technique which suppresses near-band quantization noise, without the need to correlate time delay units with the carrier frequency to form a pass band at the signal carrier.